1. Field of the Invention
The present invention relates to a controller for an electric vehicle and a driving apparatus for the electric vehicle, and more particularly to a controller and driving apparatus for an electric vehicle in which an internal combustion engine's rotational power drives a generator and the power generated by the generator drives a motor.
2. Description of the Related Art
In recent years, an increasing number of electric vehicles are running by using a motor as a driving source. They are electric automobiles, hybrid automobiles, and other environment-responsive automobiles. These environment-responsive automobiles are mainly characterized by the fact that they carry a battery, use the power stored in the battery to let the motor generate torque, and drive tires for running purposes. Recently, a permanent-magnet synchronous motor or other AC motor is employed as the motor for downsizing purposes (for achieving a high power ensity). Further, an inverter (power converter) is used to supply the DC power of the battery to the AC motor. The inverter receives the DC power from the battery and converts it to AC power. The inverter can be controlled to exercise variable speed control over the AC motor.
In the above-mentioned environment-responsive automobiles, the power supply to the inverter is always stable because they carry a battery as a power source for the motor. In such an instance, the voltage received by the inverter varies with the prevailing charge/discharge state of the battery, but is a DC voltage whose variation is generally small. A known control technology for a motor-driven system that uses a battery as a power source as described above is disclosed, for instance, by JP-B-3396440. The disclosed technology provides a plurality of motor current command tables, which include maximum and minimum values of battery voltage, and outputs an optimum motor current command in accordance with the prevailing battery voltage. This makes it possible to always generate a stable torque output in all motor rotation regions.
Recently, electric four-wheel-drive vehicles whose front wheels and rear wheels are driven respectively by an engine and by a motor have begun to be widespread in addition to hybrid vehicles. The system mounted in such four-wheel-drive vehicles incorporates no battery and drives a rear-wheel motor by using only the power generated from a generator connected to the engine with a view toward reducing the cost to a level lower than that of a mechanical four-wheel-drive vehicle and offering improved component assembly capability. In other words, the system mounted in a batteryless, electric four-wheel-drive vehicle directly drives the AC motor by using the power output from the generator. Moreover, the electric four-wheel-drive system employs an “alternator,” whose converter is a diode rectifier, as the generator in order to further reduce the cost. The electric four-wheel-drive vehicle may be such that its rear wheels are engine-driven while its front wheels are motor-driven. The electric vehicle described above is not limited to a four-wheeled vehicle and may be a vehicle having six or more wheels.